An active field of application of nuclear relaxation enhancement is the development and use of magnetic materials as contrast agents in magnetic resonance imaging. The magnetic species enhance the proton relaxation rates due to a random variation of the electron spin—nuclear spin interactions (the dipole-dipole interaction and the magnetic hyperfine interaction between the nuclear and electron magnetic moments), which open new pathways for longitudinal as well as transverse relaxation.
A category of contrast agents is paramagnetic solutions of transition metal complexes (such as Gd and Mn based contrast agents). In this case, the origin of the nuclear relaxation enhancement is found in the value of the electronic magnetic moment (about 650 times that of the proton). The efficiency of contrast agents is investigated involving a concept of relaxivity, referring to the nuclear relaxation enhancement normalized to 1 mM concentration of the magnetic species. At not too high concentration of the paramagnetic species, the enhancement is proportional to that concentration. Measurements of the relaxation enhancement or relaxivity over a broad range of magnetic fields are referred to as relaxometry, and the resulting curve is denoted as a nuclear magnetic relaxation dispersion (NMRD) profile. On the experimental side, the NMRD profiles are usually measured by the field-cycling technique, where the magnetic field is rapidly switched between different values. The measured relaxivity values for Gd and Mn based contrast agents are usually in the range of 20-50 1/mM*s. See (D. Kruk, T. Nilsson, J. Kowalewski, Phys. Chem. Chem. Phys., 3, 4907-4917, (2001), D. Kruk, J. Kowalewski, J. Biol. Inorg. Chem., 8 (5), 512-518, (2003)).
The publication Chandrasekar Rajadurai, Frank Schramm, Susan Brink, Olaf Fuhr, Robert Kruk, Mohammed Ghafari, Mario Ruben, “Spin Transition in a Chainlike Supramolecular Iron(II) Complex”, Inorg. Chem. (communication), (2006), 45, 10019-10021 describes the synthesis, structure and characterization of the spin transfer complex [FeII-(L)2H](ClO4)3.MeOH [L=4′-(4″′-pyridyl)-1,2′:6′1″-bis-(pyrazolyl)pyridine] (complex 1). The publication also describes the reversible, thermally driven spin transition at 286 K with a hysteresis loop of ca. 2 K of complex 1.
The publication of Chandrasekar Rajadurai, Olaf Fuhr, Robert Kruk, Mohammed Ghafari, Horst Hahn and Mario Ruben, “Above room temperature spin transition in a metallo-supramolecular coordination oligomer/polymer”, Chem. Commun., 2007, 2636-2638 describes the spin transition and other physical properties of the complex 1.